The present invention is directed to a method for increasing the permeability of underground hydrocarbon reservoirs containing clays and other fine particles where damage has occurred or may occur. More particularly, the invention concerns an alkaline flooding method wherein buffered, high pH solutions are employed to carry away the dissolved clays and other fines after reaction with the alkaline treating solution.
Numerous methods have been developed to improve the permeability of hydrocarbon formations, and ultimately, improve oil recovery from such formations. Various chemical and fracturing treatments represent the prior art. These techniques, however, will not work effectively in all situations.
Reservoir rock which contains clay particles is particularly susceptible to permeability damage from the clay particles. Several types of clay particles such as montmorillonite and illite, undergo substantial expansion upon contact with water. Furthermore, many types of mineral particles contained within the reservoir rock are mobile and tend to migrate within the reservoir rock and plug pores. These mobile fines reduce the permeability of the formation, forming bridges which fill and block pore constrictions. Acid and alkaline treatments are the chief measures employed to solve these permeability problems.
Acid treatments suffer from several drawbacks. Calcite cemented and weakly cemented formations tend to break down under such treatments, damaging the formation matrix. Second, permeability improvements from acid treatments in such formations may not last. Furthermore, acid treatments are often ineffective or perhaps damaging in reservoirs with high formation temperatures. At temperatures near or above 250.degree. F., the acid may react too fast to achieve any penetration into the reservoir matrix.
One method of solving permeability damage from clay swelling and clay particle migration is to consolidate the clay fines by cementing them in place. U.S. Pat. No. 3,603,399 describes the use of a hydroxy aluminum solution to stabilize the water sensitive clays in the formation by cementing them in place. A similar approach is taken by the method of U.S. Pat. No. 4,031,959 wherein a solution of alkaline earth metal hydroxides is employed with alkaline earth metal salts to consolidate the clay fines in place and prevent their migration through the reservoir. U.S. Pat. No. 4,284,140 describes the use of potassium hydroxide solutions to stabilize the clay fines. See also SPE Paper No. 11721, "Stabilizing Clays With Potassium Hydroxide" by R. D. Sydansk, presented at the California Regional Meeting held in Ventura, Calif., Mar. 23-25, 1983 and Society of Petroleum Engineers Journal, Dec. 1982, Page 910.
A different approach is taken by U.S. Pat. No. 2,679,294 which employs a high pH solution of 5% or more sodium hydroxide to react with and dissolve clays and similar materials in the reservoir rock. A major problem with this approach, however, is that the dissolved silicate fines frequently precipitate in the formation after the alkaline treating fluids are spent. The subsequent precipitation of the silicate fines can substantially reduce the effect of the permeability treatment, and in some cases, reduce permeability below the level which existed prior to initial treatment. The use of alkali metal hydroxides or alkaline earth metal hydroxides is disclosed in U.S. Pat. Nos. 1,651,311 and 4,359,093 and relates to the alteration of the crude oil properties in order to attain increased production.
Other methods of employing alkaline treating solutions are described in U.S. Pat. Nos. 3,796,266; 3,924,683 and 2,955,653. U.S. Pat. No. 3,796,266 employs an alkaline agent such as sodium hydroxide along with a guanadine salt and an anionic surfactant. The mixture of sodium hydroxide and two surfactants in one slug followed by a water drive improves flooding and salt tolerance according to the authors. Hydroxide materials are also employed in U.S. Pat. No. 3,924,683 in a huff-puff steam process. However, the amounts of hydroxide added are very small (0.6% by weight and smaller). U.S. Pat. No. 2,955,653 employs a mixture of calcium hydroxide and calcium chloride at a pH below 12.6 to control shales and gravel packs.
U.S. Pat. Nos. 3,640,343 and 3,679,001 employ alkali metal silicates such as potassium silicate or sodium silicate along with sodium and potassium chloride salts in solution at a pH below 12 to control shales.
Aqueous slugs have been employed as spacers between the injection of various treating slugs, but not to carry away silicate fines from an alkaline hydroxide treatment. U.S. Pat. Nos. 2,747,670, 3,530,937 and 3,658,131 disclose the use of alkali solutions followed by a water slug and finally a solution designed to react with the alkali solution to form a precipitate for selectively plugging areas of lower permeability. These two references disclose a means of evening out permeability by decreasing the permeability of high permeability areas within the reservoir. U.S. Pat. No. 3,396,790 is similar to the above cited patents except that the first injected solution is a high viscosity solution which reacts with a second solution to form a precipitate for selectively plugging areas of the reservoir rock.
U.S. Pat. Nos. 3,292,697 and 3,410,343 disclose surfactant flooding methods in which alkaline materials are used to stabilize the shear thickening properties of the injected fluids and thereby help to increase recovery by increasing the sweep efficiency. U.S. Pat. No. 3,292,697 discloses the injection of water, followed by ethylenediaminetetraacetic acid to complex with formation salts followed by a surfactant mixture, which contains alkaline materials in order to stabilize the shear thickening properties of this fluid, and a water slug.